Elisp: Format String

By Xah Lee. Date: 2018-09-04. Last updated: 2025-04-20.

Format

format

(format STRING &rest OBJECTS)

Convert lisp objects into a string. It takes a input string, and several other arguments of lisp objects, and output a string.

The format control string may contain placeholders %c. The placeholders are replaced by argument to format.
Different placeholders means convert the argument to a string in particular way.

(format "Name: %s, age: %d" "jane" 25)
;; "Name: jane, age: 25"

;; decimal to hex
(format "%x" 10)
;; "a"

;; hexadecimal to decimal
(format "%d" #xa)
;; "10"

Format Template Spec

%s: string. Converted by princ. 〔see Elisp: Print, Output〕
%d: number to decimal.
%o: number to octal notation.
%x: number to hexadecimal notation.
%X: like %x, but uses upper case.
%e: number to exponential notation.
%f: number to decimal-point notation.
%g: number to exponential notation or decimal-point notation, whichever uses fewer characters.
%c: integer (Codepoint (Character ID)) to character.
%S: any lisp object to lisp syntax (using prin1). 〔see Elisp: Print, Output〕

Padding

;; padding decimal, 4 digits
(format "%04d" 100)
;; "0100"

;; padding hexadecimal, 4 digits
(format "%04x" 100)
;; "0064"

Adding Sign

;; always add sign
(format "%+d" 9)
;; "+9"

(format "%+d" -9)
"-9"

;; HHHH------------------------------
;; add sign, and padding

(format "%+04d" 10)
;; "+010"

(format "%+04d" -10)
;; "-010"

Reference